The invention relates to a device for testing web-like planar structures such as films, paper, sheet metal webs or glass webs, which move at a high speed, for surface irregularities and/or inclusions. The device has a light source, preferably a laser, from which a light beam is directed onto a rotating mirror wheel and projected onto the web to be tested via a stationary mirror configured as a folding mirror for the beam path and centered over the web to be tested. The reflected or passed light is sensed by a photoelectric transducer and supplied to an evaluation station.
Systems of this type are widely used and are known, for example, from the German patent publication Nos. 33 38 802; 83 100 964; 32 48 782; 83 101 095; 32 08 042; 31 29 808; and 24 11 407. Common to all of these systems is that the testing speed is determined by the rotational speed and the number of mirror facets of the mirror wheel which scans a light spot across the surface to be tested.
Presently, conventional mirror wheels operate in the range of 15,000 to 30,000 rpm. The mirror wheels are very likely to be destroyed if they rotate at higher speeds. Conventionally, the mirror wheels are vitreous bodies having facets with a reflective coating. Usually the mirror wheels have 10 facets but wheels with up to 20 facets have also been manufactured.
A pencil-shaped light beam, conventionally a laser beam, impinges on the mirror wheel. By rotating the mirror wheel, and consequently the facets, the beam is deflected and directed over the width of the web to be tested. If the web is very large then the mirror wheel must necessarily be disposed very high above the web, a fact which leads to a significant total height of the equipment. Therefore, it has already been suggested to dispose the mirror wheel relatively close to the web to be tested and to project the scanning beam first onto a stationary mirror disposed above the web which, in turn, directs the reflected beam onto the web. The same length of the beam and the same possible scanning width result in a shorter total height of the testing device since the mirror serves as an optical folding mirror. In particular, with same beam length the use of a single folding mirror makes it possible to reduce the total height of the equipment by one half. The use of several mirrors permits an even further reduction in the total height.
On the basis of conventional speeds of 15,000 rpm and 10 facets on the mirror wheel, there are 150,000 beam scans per minute. From the viewpoint of quietness of the motor drive, on one hand, and the strength of the mirror wheel, on the other, this scanning frequency approaches the mechanical limit of the system. At the same time, this frequency sets a limit on the speed of the web to be tested since a complete testing requires a slight overlapping of the flying light spot projected onto the web in consecutive scans. The size of the light spot cannot be increased at will since irregularities onto which the flying light spot impinges affect the intensity of the transmitted or reflected light in a percentage corresponding to the spot area obscured by the irregularity. Minor irregularities result in too small a contrast, and consequently do not permit an exact evaluation, by larger light spots.